Battery longevity estimator that accounts for episodes of high current drain

ABSTRACT

System and method for estimating a remaining capacity of a battery of an implantable medical device. The implantable medical device has a battery producing a current and having a remaining battery capacity, the implantable medical device being configured to utilize a relatively low amount of the current and, in specific instances, a relatively large pulse of the current. The processor is coupled to the battery and configured to calculate an estimate of the remaining battery capacity based, at least in part, on a measured battery parameter and occurrences of the specific instances of delivery of the relatively large pulse of the current.

BACKGROUND

Implantable medical devices such as cardioverter/defibrillators arecommonly configured to treat cardiac arrhythmias by delivering highvoltage energy pulses to cardiac tissue. Implantable defibrillatorscommonly delivery therapy by way of electrodes positioned within or nearthe heart of the patient. Such therapy includes defibrillation therapy,which utilizes a sudden, high energy pulse designed to shock the heartof the patient out of a cardiac arrhythmia if and when a cardiacarrhythmia occurs. Implantable defibrillators also commonly incorporatepacing therapy, which utilizes very low energy pulses designed totrigger cardiac contractions in lieu of an adequately frequent naturalheart beat of the patient.

Implantable defibrillators commonly incorporate a power source, such asa battery, which provides operational power to the componentry of thedefibrillator, including electronics which manage the function of thedevice, monitor the condition of the patient in which the device isimplanted and deliver therapy to the patient. Many or most devicefunctions operate effectively continually, such as sensing the cardiaccondition of the patient, or frequently, such as cardiac pacing therapydelivery in certain patients, and thus account for steady, predictableand, usually, low-level drains on the battery capacity. Defibrillationtherapy, by contrast, usually occurs very infrequently in most patients,commonly with months or years between defibrillation therapy deliveries,owing to the generally infrequent occurrence of arrhythmias whichrequire treatment. As such, defibrillation therapy is, from a standpointof battery management, a large, sudden, essentially random drain on thebattery of the implantable defibrillator.

Because implantable defibrillators often provide life-sustaining therapyto the patients in which they are implanted, it may be essential to thewell-being of the patient to understand how long the battery may beexpected to last until the battery will be discharged to a point ofbeing unable to provide reliable therapy. It is known in the art thatthe terminal voltage of batteries often utilized in implantabledefibrillators typically corresponds, to some degree, to the remainingcharge in the battery. For instance, as the remaining charge decreases,the terminal voltage likewise decreases. However, the terminal voltagemay not, and often does not, correspond to the remaining charge in thebattery in a wholly straight-forward and predictable relationship.

SUMMARY

In particular, it has been discovered that, while relatively steadycurrent drain on a battery may result in a relatively predictable andaccurate relationship between terminal voltage and remaining charge, asteady current drain coupled with occasional sudden, large currentdrains may reduce the accuracy of the relationship between terminalvoltage and remaining charge. The impact on the accuracy in theprediction of remaining charge of a battery based on the relationshipbetween terminal voltage and the remaining charge may be relativelyshort in duration, but has been shown to impart some lasting impact. Asudden, large current drain on top of an underlying low-level currentdrain has been shown to skew the terminal voltage to remaining chargerelationship such that the terminal voltage consistently measuresrelatively lower than would be anticipated in view of the remainingcharge in the battery.

The skew in the terminal voltage-remaining charge relationship may, as aresult, cause a battery to be evaluated as being low on charge and inneed of replacement or recharging some time after having an actualremaining charge sufficiently low to justify such an evaluation.Moreover, the skew to the relationship between terminal voltage toremaining charge has been shown to be relatively larger soon after alarge current drain than relatively longer periods of time after thelarge current drain. Such variation may further detract from thereliability of battery measurements based only on terminal voltage.

The impact of high current drain arising from the delivery ofdefibrillation pulses has been mitigated by adjusting battery terminalcalculations in two important ways. First, incorporated into a batteryterminal calculation is an adjustment accounting for a number of timeshigh-current drain events occur. It has been determined that theadjustment may factor in both an amount of charge utilized during thehigh-current event and an original available charge of the battery at atime of manufacture or otherwise early in the life of the battery.Second, analyses of remaining charge of the battery are spread out overrelatively long periods of time to mitigate the relatively greaterinfluence of short-term variance to the terminal voltage-remainingcharge relationship.

In an embodiment, a system comprises an implantable medical device and aprocessor. The implantable medical device has a battery producing acurrent and having a remaining battery capacity, the implantable medicaldevice configured to utilize a relatively low amount of the current and,in specific instances, a relatively large pulse of the current. Theprocessor is operatively coupled to the battery and configured tocalculate an estimate of the remaining battery capacity based, at leastin part, on a measured battery parameter and occurrences of the specificinstances of delivery of the relatively large pulse of the current.

In an embodiment, the processor is configured to calculate the estimateof the remaining battery capacity further based, at least in part, onthe measured battery parameter and a number of occurrences of thespecific instance of deliver of the relatively large pulse of thecurrent.

In an embodiment, the processor is configured to calculate the estimateof the remaining battery capacity further based, at least in part, onthe measured battery parameter, and then to decrease the estimate of theremaining battery capacity based, at least in part, on the number ofoccurrences of the specific instances of delivery of the relativelylarge pulse of the current.

In an embodiment, the processor is configured to calculate the estimateof the remaining battery capacity further based, at least in part, on afunction of the number of occurrences of the specific instances ofdelivery of delivery of the relatively large pulse of the current, anamount of charge utilized in the relatively large pulse of current andan original charge of the battery.

In an embodiment, the processor is configured to calculate the estimateof the remaining battery capacity based, at least in part, on the numberof occurrences multiplied by a ratio of the charge utilized in therelatively large pulse of current and the original charge of thebattery.

In an embodiment, the processor is configured to adjust the estimate ofthe remaining battery capacity based, at least in part, on a function ofthe remaining battery capacity measured over a period of time.

In an embodiment, the period of time is at least one week

In an embodiment, the period of time is at least two weeks.

In an embodiment, the period of time is at least four weeks.

In an embodiment, the period of time is at least twelve weeks.

In an embodiment, the period of time is at least twenty-six weeks.

In an embodiment, the function is an average of a plurality of theremaining battery capacity measurements taken during the period of time.

In an embodiment, the measured battery parameter is a battery outputvoltage.

In an embodiment, the processor is a component of the implantablemedical device.

In an embodiment, the system further comprises an external devicecomprising the processor.

In an embodiment, a method is disclosed for estimating a remainingbattery capacity of a battery of an implantable medical deviceconfigured to utilize a relatively low amount of the current and, inspecific instances, a relatively large pulse of the current, the methodutilizing a processor. The method comprises the step of calculating anestimate of the remaining battery capacity based, at least in part, on ameasured battery parameter and occurrences of the specific instances ofdelivery of the relatively large pulse of the current.

FIGURES

FIG. 1 is a view of an implantable cardioverter defibrillator;

FIG. 2 is a functional schematic diagram of the implantable cardioverterdefibrillator of FIG. 1;

FIG. 3 is a graphical representation of a relationship between a chargeremaining and a terminal voltage of a battery of an implantable medicaldevice; and

FIG. 4 is a flowchart for estimating a remaining capacity of a batteryof an implantable medical device.

DETAILED DESCRIPTION

FIG. 1 is an illustration of implantable medical device 10 implanted ina patient. In the illustrated embodiment, implantable medical device 10is a cardiac defibrillator with a pacing function. The pacing functionmay treat bradycardia and may resynchronize heart 12 in conditions ofpatient heart failure. Such a defibrillator is known as a cardiacresynchronization therapy defibrillator, known in the art as a CRT-Ddevice. In various alternative embodiments, implantable medical device10 may be a cardioverter/defibrillator without a pacing function or witha pacing function but without a cardiac resynchronization feature. Inaddition, implantable medical device 10 may be any device whichincorporates high current draws from a battery. Implantable medicaldevice 10 is coupled to heart 12 by way of coronary sinus lead 14, rightatrial lead 16, and right ventricular lead 18. Connector block 20receives connectors 22, 24 and 26 positioned on the proximal ends ofcoronary sinus lead 14, right atrial lead 16 and right ventricular lead18, respectively, and provides electrical connectivity between leads 14,16, 18 and electronic circuitry within implantable medical device 10.

In the illustrated embodiment, ring electrode 28, extendable helixelectrode 30 mounted retractably within an electrode head 32, and coilelectrode 34 are positioned on right ventricular lead 18 and areelectrically coupled to an insulated conductor within right ventricularlead 18. As illustrated, right ventricular lead 18 is positioned suchthat its distal end is in the right ventricle for sensing rightventricular cardiac signals and delivering pacing or shocking pulses inthe right ventricle. The proximal end of the insulated conductors arecoupled to corresponding connectors carried by bifurcated connector 26for providing electrical connection to implantable medical device 10.

Right atrial lead 16 may include ring electrode 36 and extendable helixelectrode 38, mounted retractably within electrode head 40, for sensingand pacing in the right atrium. Right atrial lead 16 may further includecoil electrode 42 to deliver high-energy shock therapy. Right atriallead 16 may be positioned such that its distal end is in the vicinity ofthe right atrium and the superior vena cava. Ring electrode 36, helixelectrode 38 and coil electrode 42 may each be connected to an insulatedconductor within the body of right atrial lead 16. The insulatedconductor may be coupled at its proximal end to bi-furcated connector24.

Coronary sinus lead 14 may include defibrillation coil electrode 44 thatmay be used in combination with coil electrode 34 or coil electrode 42for delivering electrical shocks for cardioversion and defibrillationtherapies. Coronary sinus lead 14 may be advanced within the vasculatureof the left side of heart 12 via the coronary sinus and great cardiacvein. In various embodiments, coronary sinus lead 14 may also be includea distal tip electrode 45 and ring electrode 47 for pacing and sensingfunctions in the left chambers of the heart. Coil electrode 44 iscoupled to an insulated conductor within the body of lead 14. Theinsulated conductor may be coupled at its proximal end to connector 22.

Electrodes 28, 30, 36 and 38 may be used to form bipolar pairs. Variousones of such bipolar pairs may be referred to as a “tip-to-ring” pairs.Electrodes 28, 30, 36 and 38 may likewise be utilized individually inunipolar configuration with implantable medical device housing 46serving as an indifferent electrode, commonly referred to as the “can”or “case” electrode. Housing 46 may also serve as a subcutaneousdefibrillation electrode in combination with one or more of coilelectrodes 34, 42 and 44 for defibrillation of atria 48, 50 orventricles 52, 54 of heart 12. In various embodiments, alternate leadsystems may be substituted for the lead system of the embodiment ofFIG. 1. Leads for use with single chamber, dual chamber, or multichamberimplantable medical devices may be utilized.

FIG. 2 is a functional schematic diagram of implantable medical device10. Connection terminal 56 provides electrical connection to the housing46 for use as the indifferent electrode during unipolar stimulation orsensing. Connection terminals 58, 59 and 60 provide electricalconnection to coil electrodes 44, 42 and 34 respectively. Each ofconnection terminals 56, 58, 59 and 60 are coupled to the high voltageoutput circuit 62 to facilitate the delivery of high energy shockingpulses to heart 12 using one or more of coil electrodes 34, 42 and 44,and, in an embodiment, housing 46.

Connection terminals 64 and 66 provide electrical connection to helixelectrode 38 and ring electrode 36 positioned in the right atrium,respectively. Connection terminals 64 and 66 are further coupled toatrial sense amplifier 68 for sensing cardiac signals originating fromthe atrium of heart 12. Such signals include atrial depolarizations andare commonly recognized as P-waves in electrocardiograms. Connectionterminals 70 and 72 provide electrical connection to the helix electrode30 and the ring electrode 28, respectively. Connection terminals 70 and72 are further coupled to a ventricular sense amplifier 74 for sensingventricular signals.

Atrial sense amplifier 68 and ventricular sense amplifier 74 may takethe form of automatic gain controlled amplifiers with adjustable sensingthresholds. In an embodiment, the general operation of ventricular senseamplifier 74 and atrial sense amplifier 68 may correspond to thatdisclosed in U.S. Pat. No. 5,117,824, by Keimel, et al., incorporatedherein by reference in its entirety. When a signal received by atrialsense amplifier 68 exceeds an atrial sensing threshold, a signal may begenerated on P-out signal line 76. When a signal received by ventricularsense amplifier 74 exceeds a ventricular sensing threshold, a signal maybe generated on R-out signal line 78 to indicate a sensing of aventricular depolarization.

In an embodiment, switch matrix 80 is used to select which of electrodes28, 30, 34, 36, 38, 42, 44 are coupled to wide band amplifier 82 for usein digital signal analysis. Selection of various ones of electrodes 28,30, 34, 36, 38, 42, 44 may be controlled by microprocessor 84 viadata/address bus 86 in order to create an electrode configuration. Theelectrode configuration may be varied as desired for the varioussensing, pacing, cardioversion and defibrillation functions ofimplantable medical device 10. Signals from the electrodes selected forcoupling to bandpass amplifier 82 may be provided to multiplexer 88, andthereafter converted to multi-bit digital signals by A/D converter 90,for storage in random access memory 92 under control of direct memoryaccess circuit 93. Microprocessor 84 may employ digital signal analysistechniques to characterize the digitized signals stored in random accessmemory 92 to recognize and classify the heart rhythm employing any ofthe numerous signal processing methodologies known in the art.

In an embodiment, upon detection of an arrhythmia, data obtained fromelectrodes 28, 30, 34, 36, 38, 42, 44, including electrograms, sensedintervals and corresponding annotations of sensed events, may be storedin random access memory 92. The electrogram signals stored may be sensedfrom programmed near-field and/or far-field sensing electrode pairs. Anear-field sensing electrode pair includes, in an embodiment, a tipelectrode and a ring electrode located in an atrium 48, 50 or aventricle 52, 54, such as electrodes 36 and 38 or electrodes 28 and 30.A far-field sensing electrode pair may, in various embodiments, includeany of the following exemplary combinations: any pair of defibrillationcoil electrodes 32, 42, 44; any of defibrillation coil electrodes 32,42, 44 with housing 46; tip electrode 30, 38 with housing 46; tipelectrode 30, 38 with a defibrillation coil electrode 34, 42; or atrialtip electrode 38 with ventricular ring electrode 28. Additionalelectrode combinations may be utilized.

In various alternative embodiments, implantable medical device 10 mayutilize leads and electrodes which are positioned outside of thethoracic cavity of the patient. In such embodiments, the electrodes maysense far-field cardiac signals, in contrast to the near-field signalssensed by electrodes 28, 30, 34, 36, 38, 42, 44 positioned in or closeto heart 12. An implantable medical device 10 which incorporates leadsand electrodes outside of the thoracic cavity of the patient is known inthe art as a subcutaneous implantable cardioverter defibrillator, andmay deliver defibrillation therapy to heart 12 in a manner related tothat of conventional defibrillators described above.

Telemetry circuit 94 may receive downlink telemetry from and may senduplink telemetry to an external programmer, as is conventional inimplantable medical devices, by means of antenna 95. Data to be uplinkedto the programmer and control signals for the telemetry circuit may beprovided by microprocessor 84 via address/data bus 86. Electrogam datathat has been stored upon arrhythmia detection or as triggered by othermonitoring algorithms may be uplinked to an external programmer usingtelemetry circuit 94. Received telemetry may be provided tomicroprocessor 84 via multiplexer 88. Numerous types of telemetrysystems known in the art for use in implantable devices may be used.

Pacer timing and control circuitry 96 includes programmable digitalcounters which control the basic time intervals associated with varioussingle, dual or multi-chamber pacing modes or anti-tachycardia pacingtherapies delivered in the atria or ventricles. Pacer circuitry 96 alsodetermines the amplitude of the cardiac pacing pulses under the controlof microprocessor 84.

During pacing, escape interval counters within pacer timing and controlcircuitry 96 may be reset upon sensing of atrial and ventriculardepolarizations, i.e., P-waves and R-waves, as indicated by signals onlines 76 and 78, respectively. In accordance with the selected mode ofpacing, pacing pulses are generated by atrial pacer output circuit 98and ventricular pacer output circuit 100. The pacer output circuits 98and 100 are coupled to the desired electrodes for pacing via switchmatrix 80. The escape interval counters are reset upon generation ofpacing pulses, and thereby control the basic timing of cardiac pacingfunctions, including anti-tachycardia pacing.

The durations of the escape intervals may be determined bymicroprocessor 84 via data/address bus 86. The value of the countpresent in the escape interval counters when reset by sensed R-waves orP-waves can be used to measure R-R intervals and P-P intervals fordetecting the occurrence of a variety of arrhythmias.

Microprocessor 84 includes associated ROM in which stored programscontrolling the operation of the microprocessor 84 reside. A portion ofthe random access memory 92 may be configured as a number ofrecirculating buffers capable of holding a series of measured intervalsfor analysis by the microprocessor 84 for predicting or diagnosing anarrhythmia.

In response to the detection of tachycardia, anti-tachycardia pacingtherapy can be delivered by loading a regimen from microcontroller 84into the pacer timing and control circuitry 96 according to the type oftachycardia detected. In the event that higher voltage cardioversion ordefibrillation pulses are required, microprocessor 84 activates thecardioversion and defibrillation control circuitry 102 to initiatecharging of high voltage capacitors 104 and 106 via charging circuit 108under the control of high voltage charging control line 110. The voltageon the high voltage capacitors is monitored via voltage capacitor line112, which is passed through the multiplexer 88. When the voltagereaches a predetermined value set by microprocessor 84, a logic signalis generated on the capacitor full line 114, terminating charging. Thedefibrillation or cardioversion pulse is delivered to the heart underthe control of the pacer timing and control circuitry 96 by an outputcircuit 62 via control bus 116. The output circuit 62 determines theelectrodes used for delivering the cardioversion or defibrillation pulseand the pulse wave shape.

Battery 118 provides power to operate the electrical componentry ofimplantable medical device 10. The electrical componentry includes, butis not limited to, microprocessor 84, RAM 92, telemetry module 94, pacertiming and control 96 cardioversion/defibrillation control 102 and highvoltage charge circuit 108. In various embodiments, battery 118 isselected from conventional implantable medical device batterychemistries, including nickel-cadmium and lithium-ion, thoughalternative chemistries may be utilized.

FIG. 3 is a graphical representation of a conventional relationship 119between terminal voltage 120 of battery 118 and remaining capacity 122of battery 118. In various embodiments, battery 118 generates terminalvoltage 120 of approximately 3.2 volts when battery 118 has a fullcharge, i.e. remaining charge 122 equals a full capacity of battery 118.As remaining charge 122 of battery 118 degrades terminal voltage 120 mayalso degrade. However, terminal voltage 120 may not degrade linearlywith the degradation of remaining charge 122. In particular, battery 118may maintain terminal voltage 120 of approximately three (3) volts,plus-or-minus approximately 0.25 volts, over much of the operationallife of battery 118. In an embodiment, when the remaining charge ofbattery 118 falls below approximately 15% of its original charge, theterminal voltage may be less than approximately 2.75 volts, at whichpoint implantable medical device 10 may indicate that battery 118 is inneed of replacement, such as by replacing implantable medical device 10as a whole or by replacing battery 118 itself, or that battery 118 is inneed of recharging in embodiments which incorporate rechargingcircuitry.

According to the above estimates for remaining charge 122, averagecurrent delivered from battery 118 is computable as the change inbattery charge over time, or:

I _(ave) =dQ/dt.   Equation 1:

A measurement of terminal voltage of battery 118 at a first time may bearrived at, then, according to:

V ₁ =f(Q ₁ , Q ₁ /t ₁)   Equation 2:

And a second measurement of terminal voltage of battery 118 at a secondtime may be defined as:

V ₂ =f(Q ₂, (Q ₂ −Q ₁)/(t ₂ −t ₁)   Equation 3:

In various embodiments, terminal voltage 120 is sufficient to arrive atan adequately accurate estimate of remaining capacity 122. However,according to the relationship of Equation 3, in alternative embodiments,relatively more accurate assessments of remaining capacity 122 may bearrived at by solving for different values of charge remaining. In suchembodiments, implantable medical device 10 calculates remaining capacity122 not directly according to terminal voltage 120, but rather accordingto multiple factors pertaining to battery 118.

In an embodiment, a percentage of the inverse of remaining capacity 122,known as a “depth of discharge” is estimated according to variousbattery parameters. In the recursive formula, a prior total chargedelivered Q_(last) by battery 118 is added to an estimated chargedelivered since the previous recursive application of the formula of atotal charge delivered from battery 118 over a period of time dQ_(est)and divided by an original charge Q_(max) of battery 118 at a time at ornear a manufacture of battery 118. In an embodiment, dQ_(est) isproportional to a current delivered from battery 118 over apredetermined timeframe. Calculation of the depth-of-discharge estimatemay be represented according to Equation 4:

DOD _(est)=(Q _(last) +dQ _(est))/Q _(max)   Equation 4:

It has been discovered, however, that high-current therapy, such asdelivery of defibrillation energy, may result in a skew to thedepth-of-discharge estimate which may last throughout the life ofbattery 118. In particular, the depth of discharge estimate ascalculated by Equation 4 may be relatively too low (i.e., battery 118may have more capacity remaining than would be suggested by thedepth-of-discharge estimate) if implantable medical device 10 deliversdefibrillation therapy to heart 12, owing to an incremental change inthe chemistry of battery 118 for each defibrillation pulse delivered.Specifically, the delivery of a single defibrillation pulse may tend toskew DOD_(est) according to Equation 1 by a value approximately equal toan amount of charge delivered in a defibrillation pulse Q_(perCharge) asa percentage of original battery capacity Q_(max). Q_(perCharge) mayvary on the basis of frequency, current and duration of thedefibrillation pulse. As such, Equation 4 may be modified into Equation5 below, which may be utilized to obtain an estimated depth-of-dischargewhich accounts for a number of times N defibrillation therapy has beendelivered. In various embodiments, defibrillation therapy mayincorporate charge delivery from approximately 7.5 milliampere-hours toapproximately 10.0 milliampere-hours.

DOD _(estDefib)=(Q _(last) +dQ _(est))/Q _(max)+(N*Q _(perCharge))/Q_(max)   Equation 5:

Equation 5 may be implemented in circumstances where an battery powereddevice has high-current drain situations in which charge fromapproximately 7.5 milliampere-hours to approximately 10.0milliampere-hours is delivered over relatively short timeframes ofapproximately twenty (20) seconds or less. In alternative embodiments,the high-current drain range may be from approximately five (5)millampere-hours and higher.

Equation 5 may be applied iteratively over a time period to obtain adepth-of-discharge estimate which reflects multiple measurements ofbattery 118 characteristics over the time period. In variousembodiments, Equation 5 may be applied iteratively more than twice. Inan embodiment, Equation 5 is applied one thousand times to obtain afinal depth-of-discharge estimate, or until a stability criteria is met.In an embodiment, a stability criteria is met if an estimated terminalvoltage V_(est) of battery 118 based on the measured output current ofbattery 118 and the estimated depth-of-discharge is within a thresholdtolerance of a measured terminal voltage V_(measured). In an embodiment,the threshold tolerance is 0.1 millivolts.

In an alternative embodiment, more than one stability criteria may beapplied, including a difference between a maximum allowable currentdrain from battery 118 and the measured output current of battery 118utilized to calculate the estimated terminal voltage V_(est) above beingless than a threshold current value. In an embodiment, the thresholdcurrent value is 0.01% of the maximum allowable current. In anembodiments, the maximum allowable current from battery 118 isapproximately 0.4 amperes. In alternative embodiments, the maximumallowable current ranges from 0.3 amperes to 0.5 amperes. In anembodiment, if either the stability condition relating to measuredterminal voltage or the difference between a maximum allowable currentand the measured output current of battery 118 is met, or if the setnumber of iterations (one thousand, in the embodiment described above)is met, then the iterative application of Equation 5 is ceased.

In order to apply Equation 5 iteratively, a measurement may be taken ofcurrent delivered by battery 118 for each iteration during the timebetween the immediately preceding iteration and the current iteration.The current over time is applied as the change in charge over that timedQ_(est) which is then applied into the new iteration of Equation 5. Asreviewed above, Equation 5 may be applied iteratively with new valuesfor dQ_(est) until the stability conditions are met or the maximumnumber of iterations are met. The final, iteratively obtained value forthe depth-of-discharge estimate may be applied to estimate a chargeremaining of battery 118.

Even after an iterative estimate of the depth-of-discharge of battery118, it may be advantageous, in certain embodiments, to base decisionsregarding replacement of battery 118 not on a single, iterative estimateof the depth-of-discharge. Such an iterative estimate may be measuredover seconds or minutes, even with one thousand iterations beingconducted, and thus may nevertheless be susceptible to short-termtransients on the chemistry of battery 118 arising from high-currentdrains on battery 118. In certain past embodiments which have notrecognized the short-term nature of skew to charge remainingcalculations of implantable medical device applications, some chargeremaining estimates have been averaged over two-week periods.

However, based on contemporary battery chemistries and the presentnature of high-current deliveries by battery 118, a two-week averagingof depth-of-discharge estimates may be susceptible to short-term skewsarising out of the high-current delivery. As such, in variousembodiments, depth-of-discharge measurements are averaged over timeperiods of greater than two weeks, such as four weeks. In variousembodiments, depth-of-discharge measurements are measured overtwelve-week moving windows. In an embodiment, depth-of-dischargemeasurements are averaged over twenty-six week moving windows. As such,the moving average of the depth-of-discharge estimates may be utilizedto make decisions concerning a likely remaining time until battery 118needs to be replaced or recharged.

FIG. 4 is a flowchart of a method of estimating a remaining batterycapacity of battery 118 of implantable medical device 10. A batteryparameter is measured (400). As described above, the measured batteryparameter may include the terminal voltage of battery 118 and a currentdelivered by battery 118. An estimate of the remaining battery capacityis calculated (402) based, at least in part, on the measured batteryparameter. In various embodiments, the calculation is based on Equation4 above, with the remaining battery capacity being based on thedifference between the original charge of battery 118 Q_(max) and thedepth-of-discharge calculated in Equation 4 above. The charge remainingis then reduced (404) based, at least in part, on a number ofoccurrences of a relatively large pulse of current, such as adefibrillation pulse. In an embodiment, the effect of steps (402) and(404) is to implement Equation 5 above to determine DOD_(estDefib),which may be converted into charge remaining of battery 118.

The estimate of remaining battery capacity is averaged (406) over amoving window in order to provide an averaged estimate of remainingbattery capacity. As described above, the remaining battery capacity maybe averaged over a window of at least two weeks. In various alternativeembodiments, the window may be at least four weeks, twelve weeks ortwenty-six weeks.

Thus, embodiments of the invention are disclosed. One skilled in the artwill appreciate that the present invention can be practiced withembodiments other than those disclosed. The disclosed embodiments arepresented for purposes of illustration and not limitation, and thepresent invention is limited only by the claims that follow.

1. A system, comprising: an implantable medical device comprising abattery producing a current and having a remaining battery capacity,said implantable medical device configured to utilize a relatively lowamount of said current and, in specific instances, a relatively largepulse of said current; said system having a processor operativelycoupled to said battery and configured to calculate an estimate of saidremaining battery capacity based, at least in part, on a measuredbattery parameter and occurrences of said specific instances of deliveryof said relatively large pulse of said current.
 2. The system of claim 1wherein said processor is configured to calculate said estimate of saidremaining battery capacity further based, at least in part, on saidmeasured battery parameter and a number of occurrences of said specificinstance of delivery of said relatively large pulse of said current. 3.The system of claim 2 wherein said processor is configured to calculatesaid estimate of said remaining battery capacity further based, at leastin part, on said measured battery parameter, and then to reduce saidestimate of said remaining battery capacity based, at least in part, onsaid number of occurrences of said specific instances of delivery ofsaid relatively large pulse of said current.
 4. The system of claim 2wherein said processor is configured to calculate said estimate of saidremaining battery capacity further based, at least in part, on afunction of said number of occurrences of said specific instances ofdelivery of delivery of said relatively large pulse of said current, anamount of charge utilized in said relatively large pulse of current andan original charge of said battery.
 5. The system of claim 4 whereinsaid processor is configured to calculate said estimate of saidremaining battery capacity based, at least in part, on said number ofoccurrences multiplied by a ratio of said charge utilized in saidrelatively large pulse of current and said original charge of saidbattery.
 6. The system of claim 2 wherein said processor is configuredto adjust said estimate of said remaining battery capacity based, atleast in part, on a function of said remaining battery capacity measuredover a period of time.
 7. The system of claim 6 wherein said period oftime is at least two weeks.
 8. The system of claim 7 wherein said periodof time is at least four weeks.
 9. The system of claim 8 wherein saidperiod of time is at least twelve weeks.
 10. The system of claim 9wherein said period of time is at least twenty-six weeks.
 12. The systemof claim 6 wherein said function is an average of a plurality of saidremaining battery capacity measurements taken during said period oftime.
 13. The system of claim 2 wherein said measured battery parameteris a battery output voltage.
 14. The system of claim 2 wherein saidprocessor is a component of said implantable medical device.
 15. Thesystem of claim 2, further comprising an external device comprising saidprocessor.
 16. A method of estimating a remaining battery capacity of abattery of an implantable medical device configured to utilize arelatively low amount of said current and, in specific instances, arelatively large pulse of said current, the method utilizing aprocessor, comprising the step of: calculating an estimate of saidremaining battery capacity based, at least in part, on a measuredbattery parameter and occurrences of said specific instances of deliveryof said relatively large pulse of said current.
 17. The method of claim16 wherein said calculating step calculates said estimate of saidremaining battery capacity further based, at least in part, on saidmeasured battery parameter and a number of occurrences of said specificinstance of deliver of said relatively large pulse of said current. 18.The method of claim 17 wherein said calculating step calculates saidestimate of said remaining battery capacity based, at least in part, onsaid measured battery parameter, said method then further comprising thestep of: reducing said estimate of said remaining battery capacitybased, at least in part, on said number of occurrences of said specificinstances of delivery of said relatively large pulse of said current.19. The method of claim 17 wherein said calculating said estimate ofsaid remaining battery capacity step is further based, at least in part,on a function of said number of occurrences of said specific instancesof delivery of delivery of said relatively large pulse of said current,an amount of charge utilized in said relatively large pulse of currentand an original charge of said battery.
 20. The method of claim 19wherein said calculating said estimate of said remaining batterycapacity step is based, at least in part, on said number of occurrencesmultiplied by a ratio of said charge utilized in said relatively largepulse of current and said original charge of said battery.
 21. Themethod of claim 17, further comprising adjusting said estimate of saidremaining battery capacity based, at least in part, on a function ofsaid remaining battery capacity measured over a period of time.
 22. Themethod of claim 21 wherein said period of time is at least two weeks.23. The method of claim 22 wherein said period of time is at least fourweeks.
 24. The method of claim 23 wherein said period of time is atleast twelve weeks.
 25. The method of claim 24 wherein said period oftime is at least twenty-six weeks.
 26. The method of claim 21 whereinsaid function is an average of a plurality of said remaining batterycapacity measurements taken during said period of time.
 27. The methodof claim 17 wherein said measured battery parameter is a battery outputvoltage.